1. Field of the Invention
The present invention relates to an ultrasonic wave motor, and more particularly to an ultrasonic wave motor utilizing a rectangular wave signal for producing vibrational energy as a driving source.
2. Description of the Related Art
The principle of operation of a generally known standing wave type ultrasonic wave motor is such that a movable member is contacted under a predetermined pressure by an ultrasonic wave vibrator adapted to be excited to generate a substantially elliptic vibration, and the movable member is driven by a frictional force between the same, with each material point of the ultrasonic wave vibrator generating a substantially elliptic vibration. Such a standing wave type ultrasonic wave motor can realize high efficiency and large output since a vibration of high efficiency can be easily realized.
It is known that a stacked piezoelectric element is used as an excitation source in a floating direction of the movable member in the ultrasonic wave motor. In this type of ultrasonic wave motor, an excitation signal having a frequency different from a natural vibration frequency of the movable member is applied to the stacked piezoelectric element to drive the movable member under a non-resonant condition. Therefore, it is superior in controllability to another type ultrasonic wave motor utilizing a resonant phenomenon of vibrations in two directions.
However, as a sine wave signal is used as the excitation signal to be applied to the stacked piezoelectric element, a vibration speed of the ultrasonic wave vibrator differs from a moving speed of the movable member during a large proportion of a contact time between the ultrasonic wave vibrator and the movable member. Accordingly, a shearing strain is generated in the ultrasonic wave vibrator, and as the shearing force is larger than the frictional force, slippage is generated between the frictional surfaces to cause a loss of energy.